plan9: add mount and bind flags
plan9: add mount and bind flags

Thanks Skip Tavakkolian.

Fixes golang/go#14452.

Change-Id: I0a4ef49edae61bb37f49b1071dd5c31e88ad6c04
Reviewed-on: https://go-review.googlesource.com/19729
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golang/sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys